Recently developed radio apparatuses like mobile phones have internal antennas rather than those to be pulled out of their cases when used, like flexible whip antennas. Using internal antennas for radio apparatuses greatly facilitates their handling or holding and increases a degree of flexibility of their case design, particularly using a thinner case.
A conventional radio apparatus having an internal antenna in a small case may cause decreasing the antenna input impedance as the antenna is located very close to metallic portions of surrounding circuits within the case. That may cause a mismatch between the antenna and its feeder circuit to degrade a performance of the radio apparatus.
In such circumstances it is known that a conventional, named folded dipole antenna may be used not to decrease the antenna input impedance too much. Two or more dipole elements are placed in parallel very closely to configure a folded dipole antenna. An end of one of the dipole elements is connected to an end of the rest of them and so are the other ends, and one of them is fed at a central feeding point. It is usually configured symmetric about the feeding point. This folded dipole antenna is disclosed, for example, in the following document:
“Antenna Engineering Handbook”, pp. 112–113, Figures. 4.1 and 4.3 (in Japanese), edited by The Institute of Electronics, Information and Communication Engineers, Ohm-sha, October 1996.
A folded dipole antenna may have impedance higher than a simple dipole antenna without being folded, and the value of impedance may be adjusted by the ratio of diameters of the dipole elements placed in parallel. A dipole antenna, however, does not fit a small-sized radio apparatus, particularly one of multiple functions leaving limited space to enclose an antenna, by nature due to its size. A folded dipole antenna may be even worse due to its complicated shape.
A radio apparatus of multiple uses or of multiple standards these days is required to have an antenna of a wider range. Plural antenna elements of various resonant frequencies need to be assembled to form an antenna of multi-frequency resonance. Each antenna element is thereby required to be smaller in size and simpler in shape than, for example, the folded dipole type.
One half of a folded dipole antenna configured symmetric can be used as a monopole antenna fed at its one end and grounded at the other end. This is called a folded monopole antenna, equivalent to the folded dipole antenna in electrical features except for having half the input impedance of the folded dipole antenna, and its application to small-sized radio apparatuses is being studied. An example of such a study is disclosed in the following reference:
Sato, K. and Amano, T, “Dual-band folded antenna with two shorted points”, IEICE General Conference B-1-57 (in Japanese), The Institute of Electronics, Information and Communication Engineers, March 2004.
This “dual-band folded antenna” is configured to have two resonant frequencies as an assembly of a pair of folded monopole antenna elements of a rather low-profile type so called inverted-L, each of which has its own resonant frequency. Advantages of this configuration are showing a feature of multi-frequency resonance, fitting a radio apparatus of a thinner case, and facilitating matching impedance of the one element by short-circuiting the forward and backward paths of the other element at a point on those paths.
This configuration, however, still has a disadvantage that needs two or more folded antenna elements to be assembled, and may not fit a small-sized radio apparatus of multiple functions, particularly for present and future use, leaving limited space to enclose an antenna.